Re: [RFC PATCH 1/5] fs: introduce notifier list for vfs inode

[Chengguang Xu <cgxu519@mykernel.net>]

 ---- 在 星期四, 2020-10-15 20:32:24 Amir Goldstein <amir73il@gmail.com> 撰写 ----
 > >  > Perhaps you can combine that with the shadow overlay sbi approach.
 > >  > Instead of dirtying overlay inode when underlying is dirtied, you can
 > >  > "pre-dirty" overlayfs inode in higher level file ops and add them to the
 > >  > "maybe-dirty" list (e.g. after write).
 > >  >
 > >
 > > Main problem is we can't be notified by set_page_dirty from writable mmap.
 > > Meanwhile, if we dirty overlay inode then writeback will pick up dirty overlay
 > > inode and clear it after syncing, then overlay inode could be release at any time,
 > > so in the end, maybe overlay inode is released but upper inode is still dirty and
 > > there is no any pointer to find upper dirty inode out.
 > >
 > 
 > But we can control whether overlay inode is release with ovl_drop_inode()
 > right? Can we prevent dropping overlay inode if upper inode is
 > inode_is_open_for_write()?
 > 
 > About re-dirty, see below...
 > 
 > >
 > >  > ovl_sync_fs() can iterate the maybe-dirty list and re-dirty overlay inodes
 > >  > if the underlying inode is still dirty on the (!wait) pass.
 > >  >
 > >  > As for memory mapped inodes via overlayfs (which can be dirtied without
 > >  > notifying overlayfs) I am not sure that is a big problem in practice.
 > >  >
 > >
 > > Yes, it's key problem here.
 > >
 > >  > When an inode is writably mapped via ovarlayfs, you can flag the
 > >  > overlay inode with "maybe-writably-mapped" and then remove
 > >  > it from the maybe dirty list when the underlying inode is not dirty
 > >  > AND its i_writecount is 0 (checked on write_inode() and release()).
 > >  >
 > >  > Actually, there is no reason to treat writably mapped inodes and
 > >  > other dirty inodes differently - insert to suspect list on open for
 > >  > write, remove from suspect list on last release() or write_inode()
 > >  > when inode is no longer dirty and writable.

I have to say inserting to suspect list cannot prevent dropping,
thinking of the problem of previous approach that we write dirty upper
inode with current->flags & PF_MEMALLOC while evicting clean overlay inode.


 > >  >
 > >  > Did I miss anything?
 > >  >
 > >
 > > If we dirty overlay inode that means we have launched writeback mechanism,
 > > so in this case, re-dirty overlay inode in time becomes important.
 > >
 > 
 > My idea was to use the first call to ovl_sync_fs() with 'wait' false
 > to iterate the
 > maybe-dirty list and re-dirty overlay inodes whose upper is dirty.
 > 

I'm curious how we prevent dropping of clean overlay inode with dirty upper?
Hold another reference during iput_final operation? in the drop_inode() or something
else?


 > Then in the second call to __sync_filesystem, sync_inodes_sb() will take
 > care of calling ovl_write_inode() for all the re-dirty inodes.
 > 
 > In current code we sync ALL dirty upper fs inodes and we do not overlay
 > inodes with no reference in cache.
 > 
 > The best code would sync only upper fs inodes dirtied by this overlay
 > and will be able to evict overlay inodes whose upper inodes are clean.
 > 
 > The compromise code would sync only upper fs inodes dirtied by this overlay,
 > and would not evict overlay inodes as long as upper inodes are "open for write".
 > I think its a fine compromise considering the alternatives.
 > 
 > Is this workable?
 > 

In your approach, the key point is how to prevent dropping overlay inode that has
dirty upper and no reference but I don't understand well how to achieve it from
your descriptions.


Thanks,
Chengguang